Cleaning and passivating semiconductor surfaces, including those of the micromechanical-type, is difficult due to their architectures which may comprise spatially-convoluted, micron/submicron-scale features. A recent innovation of Texas Instruments Incorporated of Dallas, Tex. is the digital micromirror device or the deformable mirror device (collectively DMD). The DMD is an electro/mechanical/optical Spatial Light Modulator (SLM) suitable for use in displays, projectors and hard copy printers. The DMD is a monolithic single-chip integrated circuit SLM, comprised of a high density area or linear array of 16 micron square deflectable micromirrors on 17 micron centers. These mirrors are fabricated over address circuitry including an array of SRAM cells and address electrodes which generate electrostatic attraction forces. Each mirror forms one pixel of the DMD array and may be monostable, or bistable, that is to say, stable in one of two positions, wherein a source of light directed upon the mirror array will be reflected in one of two directions. In one stable "on" mirror position, incident light to that mirror will be reflected to a projector lens and focused on a display screen or a photosensitive element of a printer. In the other "off" mirror position, light directed on the mirror will be deflected to a light absorber. Each mirror of the array is individually controlled to either direct incident light into the projector lens, or to the light absorber. The projector lens ultimately focuses and magnifies the modulated light from the pixel mirrors onto a display screen and produce an image in the case of a display. If each pixel mirror of the DMD array is in the "on" position, the displayed image will be an array of bright pixels.
For a more detailed discussion of the DMD device and uses, cross reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method"; U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard Independent Digitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled "Printing System Exposure Module Alignment Method and Apparatus of Manufacture", each patent being assigned to the same assignee of the present invention and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the image is achieved by pulse-width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System", assigned to the same assignee of the present invention, and the teachings of which are incorporated herein by reference.
The DMD is revolutionary in that it is truly a digital display device and an integrated circuit solution. The evolution and variations of the DMD can be appreciated through a reading of several commonly assigned patents. The "first generation" of DMD spatial light modulators implemented a deflectable beam wherein the mirror and the beam were one in the same. That is, an electrostatic force was created between the mirror and the underlying address electrode to induce deflection thereof The deflection of these mirrors can be variable and operate in the analog mode, and may comprise a leaf-spring or cantilevered beam, as disclosed in commonly assigned U.S. Pat. No. 4,662,746 to Hornbeck, entitled "Spatial Light Modulator and Method", U.S. Pat. No. 4,710,732 to Hornbeck, entitled "Spatial Light Modulator and Method", U.S. Pat. No. 4,956,619 to Hornbeck, entitled "Spatial Light Modulator", and U.S. Pat. No. 5,172,262 to Hornbeck, entitled "Spatial Light Modulator and Method", the teachings of each incorporated herein by reference.
A "second generation" of the DMD is embodied in commonly assigned U.S. Pat. No. 5,083,857 entitled "Multi-Level Deformable Mirror Device",as well as in co-pending patent application Ser. No. 08/171,303 entitled "Improved Multi-Level Digital Micromirror Device, filed Dec. 21, 1993. In this second generation device, the mirror is elevated above a yoke, this yoke being suspended over the addressing circuitry by a pair of torsion hinges. As depicted in FIG. 3c of this application, an electrostatic force is generated between the elevated mirror and an elevated electrode. When rotated, it is the yoke that comes into contact with a landing electrode, whereby the mirror tips never come into contact with any structure. The shorter moment arm of the yoke, being about 50% of the mirror, allows energy to be more efficiently coupled into the mirror by reset pulses due to the fact that the mirror tip is free to move. Applying resonant reset pulses to the mirror to help free the pivoting structure from the landing electrode is disclosed in commonly assigned U.S. Pat. No. 5,096,279, entitled "Spatial Light Modulator and Method, and U.S. Pat. No. 5,233,456 entitled "Resonant Mirror and Method of Manufacture". However, some of the address torque generated between the mirror and the elevated address electrode is sacrificed compared to the first generation devices because the yoke slightly diminishes the surface area of the address electrode.
In operation, the deflectable mirror of the DMD, or the yoke supporting the elevated mirror, will land upon and engage a landing electrode. This movable element is subject to stiction forces, which is commonly known as the tendency of a movable element to stick to the engaged element or pad. This phenomenon is due to many known and unknown physical characteristics and features of the parts, and can include Van der Waals forces, friction, and adhesion. One solution minimizing the tendency for a micromechanical device to stick to an engaged element is to passivate the engaged element, as disclosed in commonly assigned U.S. Pat. No. 5,331,454 to Hornbeck, entitled "Low Reset Voltage Process for DMD", the teachings of which is incorporated herein by reference. An orientated monolayer passivated upon the landing electrode of the spatial light modulator element such that when the element (mirror or yoke) is activated and deflects to come in contact with the landing electrode, the orientated monolayer decreases the Van der Waals and stiction forces to reduce the tendency for the element to stick to the electrode.
As disclosed in commonly assigned co-pending patent application Ser. No. 08/239,497 entitled "PFPE coatings for Micro-Mechanical Devices", a layer of perfluoropolyether (PFPE) is passivated upon the landing electrode. PFPE is characterized as having "self-healing" tendencies whereby the molecules will migrate to repair a worn surface when the beam is not engaging the landing electrode. Using PFDA or PFPE, passivating the landing electrode has proved to remarkably reduce stiction in micromechanical devices.
The physical attributes of micromechanical surfaces, and methods for passivating the same, has proven to be a challenging and yet to be fully understood science. Simply passivating a surface may not be sufficient to adequately reduce stiction forces for the extended useful life of the device. Cleaning and preparation of the landing electrode before passivation has proven to be a critical procedure if acceptable passivation is to be achieved.
It is well known in the art to employ a supercritical fluid (SCF) to displace a non-SC liquid ambient after a wet chemical process, in effect, to dry the surface. One article authored by Gregory T. Mulhern entitled "Supercritical Carbon Dioxide Drying of Micro Structures" discusses using supercritical carbon dioxide to dry microstructures. This technique is useful for drying a surface when surface tension effects are critical. By immersing structures in liquid and then going through a transition into the supercritical region, all surface tension effects can be avoided. In an article authored by R. L. Alley, et al. entitled "The Effect of Release-etch Processing on Surface Microstructure Stiction", there is discussed reducing stiction or unwanted adhesion which occurs after release etch, rinse and dry processing. This article notes that a residue dissolved in the water and redeposited during drying is responsible for one form of adhesion, by solid bridging. A self-assembled monolayer treatment is discussed as being integrated into the post-release rinse processing to provide a durable, hydrophobic, low-energy surface coverage that should reduce the susceptibility to stiction. The films form very low surface energies to alleviate stiction. Both the articles authored by Alley, et al. and Mulhern specifically focus on releasing stuck parts which arise out of the wet chemical process employed.
In an article authored by Edward Bok, et al. entitled "Supercritical Fluids for Single Wafer Cleaning", a wafer cleaning system is discussed. A supercritical fluid, such as carbon dioxide, is cycled between two pressures so that contaminants and particles on a wafer can be effectively dislodged during the expansion phase of the pulsation. Additives may be included to modify the chemical properties, polarity, or solvating power of the fluid. For example, O.sub.2, O.sub.3, or H.sub.2 O.sub.2 may be used to oxidize the wafer surface or organic contaminants. Chemical treatments such as HF etching (wet or anhydrous) are notorious for an electrostatically active wafer surface which attracts particle and chemical contaminants. Such oxide removal or cleaning processes are usually followed by further processing to clean and passivate the wafer surface. Cleaning with a CO.sub.2 supercritical fluid alleviates the electrostatic problem because the volume of fluid between the wafer and the cleaning chamber metal walls is minimized. The system is noted as ideally suited for single wafer, cluster-tool configurations where supercritical fluid cleaning can provide a contaminant free, passivated surface between various processing steps.
In an article authored by W. Dale Spall, entitled "Supercritical carbon dioxide Precisian Cleaning for Solvent and Waste Reduction", a supercritical carbon dioxide cleaning solvent is discussed. Supercritical carbon dioxide is applied for the removal of organic compounds with mid-to-low volatilities. The enhanced solubility of organic compounds in the supercritical state forms the basis for using supercritical fluids as cleaning solvents. The low viscosity, low surface tension, high density, and high diffusion rates mean that supercritical fluids can readily penetrate into small regions to remove contaminants.
In an article authored by Theresa A. Core, et al. entitled "Fabrication Technology for an Integrated Surface-Machined Sensor", a supercritical carbon dioxide is discussed to clean the surface before a wafer drys. The supercritical phase of the fluid exhibits no surface tension, and thus will not damage the wafer surface.
In an article authored by Steven T. Walsh, et al. entitled "Overcoming Stiction in MEMS Manufacturing", the problem of surface tension because of a liquid/solid interface is discussed. To avoid stiction, it is suggested that supercritical carbon dioxide drying is suitable to solve the surface tension problem because a liquid/solid interface is never formed.
The state of the art recognizes that a supercritical fluid (SCF) is effective to remove liquids, i.e. dry, surfaces associated with wet chemical processing. Using a supercritical fluid to release components stuck by liquid-solid surface tension forces is recognized. However, it is not recognized in the art to utilize a SCF to clean a surface in preparation of passivation of a micromechanical device, which device has a component that makes repetitive contact during operation of the part with the passivated surface. Thus, the prior art addresses processing a component to remove liquid contaminants derived from wet-chemical process-related sticking, not the operational post-processing sticking which can occur in micromechanical device such as the DMD.
There is a need in the industry to preserve surface integrity after SCF cleaning, to achieve an effective passivation treatment for devices having components making repetitive contact over the life of the device. An effective passivation increases the device yield, performance, and especially, long-term reliability. In particular, there is a need to preserve a surface free of contaminants, especially organic contaminants, which can preclude an effective passivation treatment suitable for the life of the device. Simply cleaning a surface with a supercritical fluid, transferring the surface through an ambient clean room environment and then passivating the cleaned surface can degrade or compromise the entire passivation treatment due to the deposition of clean room organic contaminants. Clean rooms may have a low particle count, but contain organic and inorganic vapors, solvents, perfumes, and moisture that potentially degrade the cleaned surface prior to passivation. The presence of various organic species on surfaces exposed to clean room ambients has been reported by A. J. Muller, et al. in the reference entitled "Concentrations of Organic Vapors and Their Surface Arrival Rates at Surrogate Wafers During Processing in Clean Rooms". Water vapor at 20 Torr can potentially degrade a device surface. Thus, the present invention sets out to achieve an effective method for cleaning and passivating a semiconductor surface, especially a micromechanical device such as those of the DMD type.